One of the principal stages of constructional engineering, whether in building construction, roads, dockyards and airfields, involves binding together of various units of inert materials like stones, stone aggregates, bricks and brick aggregates, etc., with some type of cementing material. The chief purpose of this cementing material is to impart strength, rigidity, solidity, durability and such other structural requirements as desired in a particular type of construction. Except for clay which can be used directly, generally the cementing materials are not found in a ready to use form in nature. You will find that, in almost all cases, these materials have to be manufactured from raw materials.
The focus of this unit will be on cement and we will be studying their types, composition, properties, uses, manufacture, hydration tests and fields of application.
In India, Portland cement was first manufactured in 1904 near Madras by the South India Industrial Ltd. But this venture was not successful. Indian Cement Co. by 1914 was able to manufacture about 1000 tonnes of Portland cement. By 1918 three factories were established. During the First Five year plan (1951-56), cement production in India rose from 2.69 million tonnes to 4.60 million tonnes. During 1977, there were 56 cement factories in India producing a total of 19 million tonnes of cement. This increased to 20.77 million tonnes in 1981. However the decade ending 1990 saw a big boost in cement production, with figures reaching 44.88 million tonnes. This is expected to touch to 80 million tonnes by 1994-95 and cross 100 million tonnes by the end of the century.
Manufacturing of Cement
The raw materials for manufacturing of Portland cement are :
- Calcareous material – limestone or chalk
- Argillaceous material – Shale or clay
(a) Grinding of raw materials
(b) Mixing them intimately in certain proportion, depending on their purity and composition.
(c) Burning them in a kiln at temperatures of 1330oC to 1500oC, at which the material sinters and partially fuses to form modular shaped clinkers.
(d) Cooling of clinker and grinding it to fine powder with addition of 2 to 3% of gypsum.
You may note that there are two processes of manufacturing of cement
(i) Wet process
(ii) Dry process
They are so called depending upon whether mixing and grinding of raw materials is done in wet or dry conditions. In India, most of the cement factories use the wet process, though factories employing dry process have also been commissioned.
Let us have a brief insight into these processes :
This process can be understood in a sequential form as below :
(i) Limestone brought from quarry is crushed to smaller fragments.
(ii) Mixed with clay or shale; ground into a fine consistency in ball mill and converted into slurry by addition of water.
(iii) The slurry is tested for correct composition and sprayed on to the upper end of the rotary kiln. The rotary kiln is a thick steel cylinder of diameter varying from 3 to 8 metres and length varying from 30 metres to 200 metres.
(iv) The temperature in the hottest part of the kiln is about 1500oC resulting in the slurry getting converted into a fused mass of 3 mm to 20 mm size known as clinker. This clinker is cooled under controlled conditions.
(v) Finally, this clinker is ground in a ball mill with 2 to 3 percent of gypsum to produce portland cement.
In this process :
(i) The raw materials are crushed dry and fed into a grinding mill in correct proportions, where they are reduced to a fine powder.
(ii) This powder is then corrected for its composition and fed into a granulator. Water, 12 percent by weight of this powder, is added to convert it into pellets.
(iii) These pellets are then ground to produce cement.
(iv) The dry process is considered to be economical as compared to wet process because of less consumption of fuel in the kiln.
The raw materials used for the manufacture of cement consist predominantly of lime, silica, alumina and iron oxide. These oxides interact with each other during the process of burning in the kiln to form more complex compounds. Approximate oxide composition of ordinary Portland cement are given in Table 2.1.
- The ratio ofis not greater than 1.02 and is not less than 0.66.
- The ratio of percentage of Alumina/Iron oxide is not less than 0.66.
- Insoluble residue percent by mass is not greater than 4 percent.
- Weight of magnesia is not greater than 6 percent.
- SO3 content is not greater than 2.5 and 3.0 when tricalcium aluminate percent by mass is 5 or less and greater than 5 respectively.
- Total loss on ignition is not greater than 5 percent.
Quick Setting Cement
You may recall that we had stated that gypsum is added at the time of grinding to prevent flash set of cement. In quick setting cement, the early setting is brought about by reducing this gypsum content. This cement is, therefore, required to be mixed, placed and compacted quickly.
• Quick setting cement is used mostly in underwater construction, where pumping of concrete is involved, resulting in time saving and economy.
• This cement may also be used to advantage in some typical grouting operations.
Super Sulphate Cement
We have seen earlier in Portland Blast Furnace slag cement that due to use of granulated slag it possesses better resistance to sulphate attack. In super sulphate cement, this property is made use of extensively by grinding together a mixture of 80-85 percent granulated slag, 10-15 percent hard burnt gypsum and about 5 percent Portland cement clinker. This is ground finer than OPC. The specific surface must not be less than 4000 cm2 per gm. Super sulphate cement has low heat of hydration of about 45-50 calories per gm at 28 days and possesses high sulphate resistance.
An important point to be noted by you is that when we use super sulphate cement the water/cement ratio should not be less than 0.5 and wet curing for not less than 3 days after casting is essential as premature drying out results in an undesirable or powdery surface layer. A mix leaner than 1 : 6 is also not recommended.
• Super sulphate cement is particularly recommended for use in foundation where chemically aggressive conditions exist.
• As super sulphate cement has more resistance than Portland blast furnace cement to attack by sea-water, it is also used in marine works.
• In fabrication of reinforced concrete pipes to be used in sulphate bearing soils.
Low Heat Cement
While discussing heat of hydration we had pointed out that the reaction of cement with water is exothermic, resulting in liberation of considerable quantity of heat. We also know that it is the reactions with C3S and C3A which produce most heat. Therefore in low heat cement, the contents of C3S and C3A are reduced and C2S is increased.
A reduction of temperature so obtained retards the chemical action of hardening and so further restricts the rate of evolution of heat. Thus, the evolution of heat extends over a long period. Therefore, low heat cement has slow rate of gain of strength, but its ultimate strength is same as that of OPC. The heat of hydration of low heat cement shall be 7 days - not more than 65 calories per gm
28 days - not more than 75 calories per gm
• Because of low and slow rate of evolution of heat, low heat cement is ideally suited for use in mass concrete construction such as Dams.
Portland Pozzolana Cement
Let us first examine what is a Pozzolana. A Pozzolana is essentially a siliceous material which, while in itself possessing no cementitious properties will, in finely divided form and in the presence of water, react with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. The Pozzolanic materials used in the manufacture of Portland pozzolana cement may include such natural materials as
(a) Diatomaceous earth,
(b) Opaline Cherts an Shales,
(c) Tuffs, and
(d) Volcanic ashes and pumicites.
In addition, materials processed by calcinations of or fly-ash etc., are also used. Portland pozzolana cement is produced by grinding together Portland cement clinker and Pozzolana with addition of gypsum. The pozzolona content shall not be less that 10 percent and not more than 25 percent by weight of portland pozzolona cement. The specific surface of pozzolanic cement shall not be less than 3000 cm2/gm. The compressive strength is specified to be not less than 220 kg/cm2 at 7 days and not less than 310 kg/cm2 at 28 days. In India, there is apprehension in the minds of the user to user to use this cement in constructional work. But then this fear is not justified as it is not inferior to O.P.C in anyway except for rate of development of strength upto 14 days. It should also be cured under moist conditions for a sufficient period.
• Portland pozzolona cement can generally be used where ordinary Portland cement is usable.
• Since it reduces the leaching of calcium hydroxide, therefore it is particularly useful in marine and hydraulic structures.
Air Entraining Cement
This cement is made by mixing a small amount, 0.025 to 0.1 percent by weight of an air entraining agent with ordinary Portland cement clinker at the time of grinding. Some of these air entraining agents are :
(a) Alkali salts of wood resins.
(b) Synthetic detergents of the alkayl-aryl sulphonate type.
(c) Calcium ligno sulphate derived from the sulphate process in paper making.
(d) Calcium salts of glues and other proteins obtained in the treatment of animal hides.
• Air en-trained cement is ideal for use in structures subjected to freezing and thawing.
• Its use in improving work ability of cement needs to be practiced increasingly.
Masonry Cement (IS : 3466-1967)
This cement is made with such combination of materials that, when it is used for making mortar, it incorporates all good properties of lime mortar like work ability, water retention, extensible etc. and discard not so ideal properties of cement mortar like shrinkage etc. Some of the additional materials are limestone, clay, chalk, talc, water repellent materials and gypsum.
• Mostly used for masonry construction in brick or block masonry.
You will notice that concrete made with ordinary Portland cement shrinks while setting due to loss of free water. Concrete also shrinks continuously for long time. This is known as drying shrinkage. But then there are situations where this affects the functional efficiency of a structure. For example if cement used for grouting anchor bolts in machine foundations or the cement used in grouting the pre-stress concrete ducts, shrinks, then the purpose for which it has been used gets defeated. Therefore, a cement which does not shrink while hardening and thereafter, has been developed by using an expanding stabilizer very carefully. Generally, about 8 to 20 parts of sulphoaluminate are mixed with 100 parts of Portland cement clinker and 15 part of stabilizer. Curing must be carefully controlled since expansion occurs only as long as concrete is moist.
One type of expansive cement is known as shrinkage compensating cement. This cement when used in concrete, with restrained expansion induces compressive stress which more or less offset the tensile stress induced by shrinkage.
Another type is known as self stressing cement. This induces significant compressive stress after compensating the shrinkage stress, also gives some sort of prestressing effect in the tensile zone of a flexural member.
A popular non-shrinking grout developed by Associated Cement Co. Ltd is known as Shrinkkomp.
• The major use of expansive cement is for grouting machine base plates, anchor bolts, rock bolting and grouting of pres-tress ducts.
Oil Well Cement
As you are aware, oil production has become extremely important for India to improve its balance of payment position and to cut down on imports. Oil wells are drilled through stratified sedimentary rocks through great depths. Oil when struck, could escape together with gas, through the space between the steel casing and the rock formation. To prevent this, cement slurry is used. The cement slurry has to be pumped in position at considerable depth where the prevailing temperature may be 175oC, coupled with pressures upto 1300 kg/cm2. The slurry should remain sufficiently mobile to be able to flow under such conditions for several hours and then harden fairly and rapidly. In addition, it may have to resist corrosive actions because of sulphur gases or waters containing dissolved salts.
The type of cement suitable for such situations is called oil well cement. The desired properties are obtained either by adjusting the compound composition of cement or by adding retarders to the OPC. The most common agents are starches or cellulose products or acids. These retarding agents prevent quick setting and impart mobility to slurry to facilitate penetration of all fissures and cavities.
High Strength Cement
In construction engineering, there are special situations which demand use of high strength concrete as in precast concrete, prestressed concrete and air-fields, runways and taxi tracks. For this purpose, cements having much higher strength than OPC are required and are known as high strength ordinary Portland cement covered in IS : 8112-1989. The same is now called 43 grade OPC. Another high strength cement called 53 Grade is covered under IS : 122 69/1987.
The compressive strength for 43 Grade OPC and 53 Grade OPC are given in Table 2.3.